A number of metals including aluminum, lead, magnesium, zinc, zirconium, titanium, and silicon can be produced by electrolytic processes. Each of these electrolytic processes employs an electrode in a highly corrosive environment.
One example of an electrolytic process for metal production is the well-known Hall-Heroult process producing aluminum in which alumina dissolved in a molten fluoride bath is electrolyzed at temperatures of about 960° C.-1000° C. As generally practiced today, the process relies upon carbon as an anode to reduce alumina to molten aluminum. The carbon electrode is oxidized to form primarily CO2, which is given off as a gas. Despite the common usage of carbon as an electrode material in practicing the process, there are a number of disadvantages to its use, and so, attempts are being made to replace them with inert (not containing carbon) anode electrodes made of for example a ceramic, metal-ceramic “cermet” or metal containing material.
Ceramic and cermet electrodes are inert, non-consumable and dimensionally stable under cell operating conditions. Replacement of carbon anodes with inert anodes allows a highly productive cell design to be utilized, thereby reducing costs. Significant environmental benefits are achievable because inert electrodes produce essentially no CO2 or fluorocarbon or hydrocarbon emissions. Some examples of inert anode compositions are found in U.S. Pat. Nos. 4,374,761; 5,279,715; 6,126,799; 6,372,119; 6,416,649; 6,423,204; and 6,423,195, all assigned to Alcoa Inc. and herein incorporated by reference.
Although ceramic and cermet electrodes are capable of producing aluminum having an acceptably low impurity content, they are susceptible to cracking during cell start-up when subjected to temperature differentials on the order of about 900° C.-1000° C. In addition, ceramic components of the anode support structure assembly are also subject to damage from thermal shock during cell start-up and from corrosion during cell operation. One example of an inert anode assembly for an aluminum smelting cell is shown in FIG. 3 of U.S. patent application Publication 2001/0035344 A1 (D'Astolfo, Jr. et al.) where cup shaped anodes can be filled with a protective material and then attached to an insulating lid or plate.
Making a low resistance electrical connection between a ceramic or ceramic-metallic electrode and a metallic conductor has always been a challenge. The connection must be maintained with good integrity (low electrical resistance) over a wide range of temperatures and operating conditions. Various attempts have been made with brazing, diffusion bonding, and mechanically connecting with limited success. Examples of sinter threading and electromechanical attachment are shown, for example, in U.S. Pat. Nos. 4,626,333 and 6,264,810 B1 (Secrist et al, and Stol et al. respectively). Also, differential thermal growth between the pin and ceramic or cermet, over the assembly and process temperature range can cause the inert material to crack and/or the electrical connection to increase in resistance; rendering the assembly unfit for continued use.
What is needed is a pin-to inert material interior connection that is simple, not labor intensive to assemble and which will provide a low electrical resistance connection that will not deteriorate over time or cause cracking of the anode. It is a main object of this invention to provide a low electrical resistance connection of the pin conductor and inert anode electrode. It is another object to reduce assembly costs and provide a simplified design and method.